1. Field of the Invention
The present invention relates to a magnetic developer to be used in a developing assembly to develop a latent image formed on an image bearing member such as an electrophotographic photosensitive member or an electrostatic recording dielectric. It also relates to a method of magnetic-ink character recognition to read and recognize the magnetism of magnetic ink characters printed with the magnetic developer. The present invention also relates to a magnetic developer suited for printing characters having magnetism, used in a magnetic-ink character recognition system.
2. Related Background Art
The magnetic developer of the present invention can be preferably used in an electrophotographic image forming process as a magnetic developer for converting a digital latent image into a visible image by a reversal development system, in which the latent image is expressed by unit picture elements and the unit picture elements are represented by on-off binary or finite gradation.
Conventionally, a developing system using a magnetic toner serving as a one-component developer to convert a latent image formed on a photosensitive drum serving as an electrostatic latent image bearing member into visible images is as follows. Toner particles are imparted with electric charges with a polarity opposite to that of the development standard potential and the electrostatic image formed on the photosensitive drum by friction between magnetic toner particles themselves and between a sleeve serving as a developer carrying member and magnetic toner particles, and the magnetic toner particles thinly spread on the sleeve are transported to the developing zone between the photosensitive drum and the sleeve, and in the developing zone the magnetic toner is propelled to the latent image by the action of a magnetic field formed by a magnet fixed inside the sleeve, thus converting an electrostatic latent image on the photosensitive drum into a visible image.
In recent years, with the popularization of image forming apparatus such as electrophotographic copying machines, their use has spread over various fields. With such a background, a printer for the magnetic-ink character recognition (hereinafter abbreviated "MICR") system has been proposed as an application of electrophotographic printers.
The MICR system is a system mainly devised to efficiently assort checks and bills using a magnetic character reader in clearing houses, on which information data such as bank names, amounts and account numbers have been printed with a magnetic ink. Offset printing using a magnetic ink has been hitherto prevailing. However, as commercial transactions of personal checks, bills and so forth have become active, there is an increasing demand for small-sized printers of MICR characters (hereinafter simply called "MICR encoder(s)".
As conventional small-sized MICR encoders, impact printers applying a thermal transfer system are prevalent. Most of such printers, however, are mono-functional machines printing only MICR characters, and can not be utilized in the preparation of conventional documents. Hence, further improvement has been pursued.
It is sought to provide an electrophotographic printer that can print conventional documents and/or graphics and as well as MICR characters with a good MICR recognition rate. When the electrophotographic printer is applied to MICR encoders using a conventional magnetic developer, the accuracy (recognition rate) in the magnetic character reading by an MICR reader-sorter is very poor, compared with offset printing or printing with the impact printer. Thus, its use is not practical.
Securities on which the MICR characters have been printed are usually passed through the MICR reader-sorter about ten times on average. Every time they pass through the machine for magnetic character reading, they rub against the magnetic head at, a high speed. Hence, the magnetic developer used for printing the MICR characters is required to inhibit smearing or falling-off of characters as a result of rubbing.
For the MICR characters, there is a standard called E-13B, prescribed in ANS (American National Standard) x9.27-198x or JIS C6251-1980. The E-13B standard is constituted of numbers of 0 to 9 and four kinds of symbols. Bank codes, branch office codes, account numbers, amounts and so forth are printed on securities in combinations of such numbers and symbols.
In order to improve the recognition rate of the MICR reader-sorters, the shape and size of printed MICR characters are required to be reproduced in a high precision, and characters must be finely and faithfully reproduced without crushed or broken line images.
In order to achieve a high recognition rate in MICR character printing using the electrophotographic printer, it is necessary to use a magnetic developer containing a specific magnetic material having magnetic properties different from magnetic materials used in conventional magnetic developers.
That is, it is necessary to use a magnetic material showing a relatively large residual magnetization .sigma.r.
The magnetic developer is also required to Show as good triboelectric chargeability as the conventional magnetic developers of conventional electrophotographic printers, as well as uniform coating ability on the developer-carrying member (hereinafter "sleeve") of the developing assembly. To meet such requirements, the permeability of a magnetic material contained in the magnetic developer is also important.
Japanese Laid-Open Patent Application No. 59-7379 discloses a magnetic toner containing a cobalt-substituted triiron tetraoxide powder with a length/breadth ratio (major axis/minor axis ratio) of 1 to 5 and having a residual magnetization of 10 to 20 emu/g and a coercive force of 150 to 450 oersteds. It, however, is difficult for this toner to form a uniform toner layer on the sleeve, and the toner has a poor triboelectric chargeability and may give a low image density and a poor sharpness.
Japanese Laid-Open Patent Application No. 63-108354 discloses an insulating magnetic capsule toner containing a spherical magnetic powder having a length/breadth ratio of 1 to 1.5 and a permeability of 3.80 to 6.00. Japanese Laid-Open Patent Application No. 59-20484 also discloses a magnetic toner containing a ferromagnetic powder having a maximum permeability of 3.95 to 5.50. These toners can achieve a high image density and are preferable, but further improvement is required to meet higher requirements for resolution and adaptability to reversal development systems.